Current unit for arc welding

ABSTRACT

An a.c. operated arc welding current supply unit comprises a frequency converter of the series capacitor type and operating with a half period which is substantially less than the average duration of the current and voltage transients caused by short circuits through droplets of weld material during welding. The frequency converter is connected to welding electrodes through a transformer in series with a rectifier to provide direct current for the welding electrodes, and the frequency converter is associated with a control device for controlling the operating frequency of the converter in a manner such that it is substantially inversely proportional to the square of the input voltage of the current supply unit, thereby maintaining the power output of said unit substantially unchanged irrespective of changes in load caused by the welding operation and irrespective of variations in said input voltage.

This is a divisional application of Ser. No. 795,501, filed May 10,1977, now abandoned.

The present invention relates to an arc welding current supply unitwhich is arranged to be fed with alternating current and to providedirect current for welding electrodes.

An object of the invention is to provide a novel and useful weldingcurrent supply unit which will facilitate welding operations so that anacceptable weld can be made by relatively unskilled persons and whichwill also enable more satisfactory welding operations to be carried outthan was hitherto possible, with the use of conventional welding currentunits operating at main frequencies.

To this end it is suggested in accordance with the invention that an arcwelding current supply unit of the aforementioned type includes acontrolled frequency converter operating with a half-period which isless than the average duration of the current and voltage transientscaused by short circuits through droplets of the weld material, e.g.less than 3 milliseconds, and prefeably less than 1.5 milliseconds, andadapted to be connected to the welding electrodes through a transformerin series with a rectifier, and also includes a control device which isadapted to control the converter in a manner such that the arc powerremains substantially unchanged irrespective of changes in load causedby the welding operation, wherein said converter is of theseries-capacitor type converter, i.e. the primary winding of thetransformer is supplied from a d.c. voltage intermediate stage bycontrolled alternating .Iadd.charge and .Iaddend.discharge of one ormore capacitors connected in series with said primary winding, andwherein the voltage of the a.c. supply applied to the unit issubstantially constant, the control device may be adapted to control theconverter in a manner such that it operates at a constant frequencywhich can be adjusted to a desired value. On the other hand, the controldevice is arranged to control the operating frequency of the converterso that it is substantially inversely proportional to the square of thevoltage of the a.c. supply or the voltage applied to the capacitor orcapacitors.

With such a welding current supply unit there is obtained a paticularlystable and quiescent arc, irrespective of small variations in thedistance between electrode and work piece. In addition, in the event ofa short circuit caused by droplets of welding material, the arc will besmoothly re-ignited with small dynamic effect on the molten material.

So that the invention will be more readily understood and optionalfeatures thereof made apparent, an embodiment thereof will now bedescribed with reference to the accompanying schematic drawings, inwhich:

FIG. 1 is a circuit diagram of an a.c. operated arc welding currentsupply unit having a frequency converter of a type which can be used toadvantage in conjunction with the present invention;

FIG. 2 is a circuit diagram of a control circuit intended for use withthe current supply unit of FIG. 1;

FIG. 3 illustrates curves denoting voltages and currents which occurduring the normal operation of the current supply unit.

The current supply unit shown in FIG. 1 is connected at 10 to a 3-phasea.c. network. The input current is rectified in a six-element full-waverectifier 11, the rectified output voltage on lines 12, 13 beingsmoothed by a buffer capacitor 14 and applied to a frequency converterhaving--as a consequence of the shown arrangement of elements 11 to14--a low input impedance.

With the illustrated embodiment the switching elements of the frequencyconverter comprise thyristors 15, 16 which are controlled so as to bealtenately energized. The frequency converter is associated with atransformer generally shown at 17 and the primary winding 18 of which isconnected in series with load capacitors 19, 20 forming parts of thefrequency converter. The secondary winding 21 of the transformer 17 isconnected via a bridge rectifier 22 and a choke 29 to welding electrodeterminals 24, 25, which terminals are capable of being connected to awinding electrode holder and a work piece to be welded. With theillustrated embodiment there is connected between the terminals 24, 25 acapacitor 26 which is used to maintain a desired open-circuit voltage. Ashunt 27 may also be arranged for measuring the load current.

In the case of a fluctuating supply voltage and a constant converterfrequency the desired converter provides a power output which variessubstantially proportionally to the square of the supply voltage U₁₀between the terminals 10. If the power output from the current unit isto remain substantially constant irrespective of fluctuations in thesupply voltage, the frequency of the converter must be varied inproportion to 1/U₁₀ ².

In FIG. 2, there is shown a control device which is adapted to controlthe frequency converter in a manner such that the arc power remainssubstantially constant irrespective of changes in load caused by thewelding operation and by changes in input voltage. This control deviceincludes a potentiometer 33 and an oscillator 34, said potentiometerbeing adapted to control the operating frequency of the oscillator 34.The potentiometer 33 is connected between earth and a negative voltagesource which varies substantially inversely proportional to the squareof the voltage applied to the frequency converter.

In the shown embodiment, the control device further includes a fixednegative potential 335 and a load resistor 331 connected in series withthe potentiometer 33. The supply voltage U₁₀ is applied to the primaryof a transformer 334 the secondary of which is connected through abridge rectifier 333 to a smoothing capacitor 332 and said load resistor331. The values of the components are chosen so that the voltage acrossthe resistor 331 is approximately twice that of the voltage across thepower adjusting device 33. The voltage across the resistor 331 willtherefore vary in proportion in U₁₀ which in turn means that the voltageacross the power adjusting device 33 will vary approximately inproportion to 1/U₁₀ ² as long as the variations of the supply voltageare small, e.g. not more than ±5%. The frequency of the oscillator 34controlled by the voltage adjusting device 33 will thus also vary inproportion to 1/U₁₀ ². Instead of using U₁₀ as the control voltagesignal, it is of course possible to use the voltage applied to theconverter capacitor 19,20.

The oscillator 34 includes an integrating circuit comprising anamplifier 340, resistors 341 and 342, and a capacitor 343. Theoscillator also includes a level discriminator 344 with associatedcomparison resistors 345, 346, the resistor 345 being connected to theoutput of the integrating circuit and the resistor 346 being connectedto a voltage source having a fixed negative voltage. The amplifier 340is controlled in the positive sense by control signals from thepotentiometer 33 and in the negative sense by signals from the Q-outputof a monostable flip-flop 50 hereinafter described. A voltage adjuster35 in the form of a potentiometer is provided for adjusting the desiredmaximum output voltage from the current supply unit, which voltageadjuster 35 is connected via comparison resistors 36 to the teminal 25of the current supply unit and to an amplifier 37 which serves as alevel discriminator.

Control of the state of the thyristors 15, 16 is effected by means of asensing circuit comprising a transformer 38, the primary side of whichis connected via diodes 39, 40 to the anodes A₁, A₂ and cathodes K₁, K₂of the thyristors 15, 16. One end of the secondary winding of thetransformer 38 is earthed and the other end is connected to a comparisoncircuit which comprises two resistors 41, 42, the resistor 42 having oneend thereof connected to a constant negative voltage. The junctionbetween the resistors 41, 42 is further connected to an amplifier 43which serves as a level discriminator and the change-over point of whichis determined by the resistors 41, 42 and the aforementioned constantnegative voltage. Each of the amplifiers 344, 37 and 43 is connected toa respective input 45, 46, 47 of an AND gate 48, which operates in aknown manner. Thus, in order for the AND gate 48 to emit an outputsignal, it is necessary for the output signal obtained from theamplifier 344 to be positive. Correspondingly, it is necessary for theoutput signal from the amplifier 37 to be positive, i.e. for the loadvoltage on the terminal 25 to be less than the value for which theadjuster 35 is set. Finally, the output signal from the amplifier 43must be positive, which means that the anode voltage of one of thethyristors 15 or 16 is negative in relation to the cathode, which inturn means that both the thyristors 15 and 16 are de-energized.

The output of the AND gate 43 is connected to the input of themonostable flip-flop 50 which has a predetermined pulse periodcorresponding to the recovery time of the thyristors 15, 16, for example30 μus. Thus, on the output Q of the flip-flop 50 there is obtained apositive voltage pulse for a period of time corresponding to therecovery time of the thyristors 15, 16. This pulse is transmittedthrough the resistor 342 to the amplifier 340 and is also transmitted tothe trigger input T of a JK flip-flop 51, so that the JK flip-flop 51changes the status of its outputs at the end of the pulse obtained fromthe flip-flop 50. The outputs Q, Q of the JK flip-flop 51 are connectedto the base electrodes of respective transistors 54, 55 via capacitors52, 53. The emitters of the transistors 54, 55 are connected to earthwhile their collectors are connected to the primary windings ofrespective ignition transformers 56, 57 for the thyristors 15, 16. Theother ends of the primary windings are connected to a terminal having apre-determined positive potential, and the ends of the secondarywindings are connected to the cathode K₁ or K₂ and the gate G₁ or G₂ ofa respective thyristor 15 or 16. The signals from the outputs of the JKflip-flop 51 will alternatively render the transistors 54, 55 conductivevia the capacitors 52, 53, for a short period of time determined by thecapacitors 52, 53, so that the thyristors 15, 16 alternately receive ashort ignition pulse and thereby alternately supply current to theprimary winding 18 of the transformer 17 to produce an alternatingcurrent whose frequency is determined by the signals on the inputs 45,46, 47 of the AND gate 48. With this arrangement a desired substantiallyconstant arc power can be set solely by a corresponding adjustment ofthe potentiometer 33.

FIG. 3 shows the voltages U₆₀, U₆₁ at points 60, 61 and the voltage U₁₈across the primary winding 18 of the transformer 17 with normal load onthe output of the current supply unit shown in FIGS. 1 and 2. FIG. 3also shows the output current I₂₅ through the terminal 25 and the outputcurrents I₁₅ and I₁₆ (the latter shown in dash lines) from thethyristors 15, 16. In FIG. 3 the reference t₁ illustrates the point oftime when the thyristor 15 is ignited, t₂ the point of time when thethyristor 15 is ignited, t₂ the point of time when the thyristor 15 isde-energized and obtains a negative voltage between the anode A₁ andcathode K₁ as a result of the resonant circuit formed by the primarywinding 18 of the transformer 17 and the capacitors 19, 20, t₃ the pointof time when the thyristor 16 is ignited, and t₄ the point of time whenthe thyristor 16 is de-enegized and obtains a negative anode voltage asa result of said resonant circuit 18, 19, 20. The reference t₅ shows thepoint of time at which the thyristor 15 is re-ignited, whereupon thesequence is repeated provided that the load remains substantiallyunchanged. .Iadd.When thyristor 15 is turned on it provides a chargingpath for capacitor 20 and a discharge path for capacitor 19. Whenthyristor 16 is turned on and thyristor 15 is off, a charging path isprovided for capacitor 19 and a discharge path is provided for capacitor20. Each capacitor is alternately charged and discharged through theswitching thyristors and the primary winding 18. .Iaddend.

The invention is not limited to the frequency converter illustrated anddescribed, but may be used in conjunction with other frequencyconverters, for example frequency converters having forced commutationor d.c. controlled intermediate stages. If larger fluctuations in supplyvoltage U₁₀ than e.g. ±5% are likely to occur, there may be added to thevarying negative voltage source described with reference to FIG. 2 amultiplying circuit which generates across the resistor 331 a voltagewhich is truly proportional to 1/U₁₀ ².

I claim:
 1. An arc welding current supply unit arranged to be fed withalternating current voltage and to provide direct current for weldingelectrodes, said unit including a controlled frequency converteroperating with a half period which is less than the average duration ofthe current and voltage transients caused by short circuits throughdroplets of the weld material and adapted to be connected to the weldingelectrodes through a transformer in series with a rectifier, a controldevice for controlling said converter in a manner such that the arcpower remains substantially unchanged irrespective of changes in loadcaused by the welding operation, wherein current pulses are formed inthe secondary winding of the transformer by alternatingly charging anddischarging at least one capacitor which is connected in series withsaid primary winding of said transformer and which is dischargedtherethrough, and said at least one capacitor being charged by a directcurrent voltage intermediate source and being connected to said sourceby a number of thyristors equal in number to the number of saidcapacitors, said thyristors being connected to said control device, andwherein said control device further comprises means for alternatinglymaking said thyristors conductive at a frequency which is substantiallyinversely proportional to the square of said voltage applied to saidcurrent supply unit. .Iadd.
 2. An arc welding current supply unitarranged to be fed with alternating current voltage and to providedirect current for welding electrodes, said unit including a controlledfrequency converter operating with a half period which is less than theaverage duration of the current and voltage transients caused by shortcircuits through droplets of the weld material and adapted to beconnected to the welding electrodes through a transformer, a controldevice for controlling said converter in a manner such that the arcpower remains substantially unchanged irrespective of changes in loadcaused by the welding operation, wherein current pulses are formed inthe secondary winding of the transformer by alternatingly charging anddischarging at least one capacitor which is connected in series withsaid primary winding of said transformer and which is dischargedtherethrough, a direct current voltage intermediate source, andswitching means responsive to said control device for connecting said atleast one capacitor to said intermediate source for charging anddischarging said capacitor; and wherein said control device furthercomprises means for controlling said switching means to altenatinglycharge and discharge said at least one capacitor at a frequency which issubstantially inversely proportional to the square of said voltageapplied to said current supply unit. .Iaddend..Iadd.
 3. An arc weldingcurrent supply unit as claimed in claim 2 further comprising a rectifierconnected between said transformer and said welding electrodes..Iaddend. .Iadd.
 4. An arc welding current supply unit as claimed inclaim 3 wherein said control device comprises,a voltage controlledoscillator for providing a pulse train output at a rate dependent upon avoltage input thereto, said pulse train output determining the frequencyof switching of said switching means, and means connected between saidinput alternating current voltage and said oscillator for supplying tosaid oscillator a voltage input substantially inversely proportional tothe square of the voltage applied to said current supply unit..Iaddend..Iadd.
 5. An arc welding current supply unit as claimed inclaim 4 wherein said switching means comprises first and secondthyristors and wherein said control device further comprises, thyristorignition circuitry for altenately igniting said thyristors into theirconducting states in response to triggering pulses applied thereto, andgating means connected to said voltage controlled oscillator and saidthyristor ignition circuitry for applying pulses in said pulse trainoutput to said thyristor ignition circuitry as said triggering pulseswhen said gating means is not otherwise blocked. .Iaddend. .Iadd.
 6. Anarc welding current supply unit as claimed in claim 5 wherein saidcontrol device means further comprises, a first blocking means connectedbetween said thyristors and said gating means for blocking said gatingmeans at all times other than when a reverse bias appears across theanode cathode path of either of said thyristors, whereby neitherthyristor can be turned on until the previously conducting thyristor isturned off. .Iaddend..Iadd.
 7. An arc welding current supply unit asclaimed in claim 6 wherein said control device means further comprises,a second blocking means for blocking said gating means when the voltageto said welding electrodes exceeds a predetermined maximum..Iaddend..Iadd.
 8. An arc welding current supply unit as claimed in anyof claims 5, 6, or 7 further comprising first and second capacitorsalternately charged and discharged through said primary winding by thealternate conduction of the second and first of said two thyristors,wherein said first and second thyristors are in series with one anotherand connected across said d.c. voltage intermediate source, said one andsecond capacitors are in series with one another and connected acrosssaid d.c. voltage intermediate source, and said primary winding isconnected between the junction of said first and second thyristors andthe junction of said one and second capacitors. .Iaddend. .Iadd.
 9. Anarc welding current supply unit as claimed in claim 2 wherein saidswitching means comprises first and second thyristors and wherein saidat least one capacitor comprises first and second capacitors, saidthyristors, capacitors and primary winding being connected such thatsaid first capacitor is charged via said second thyristor and saidprimary winding and is discharged via said first thyristor and primarywinding, and wherein said second capacitor is charged through said firstthyristor and primary winding and is discharged through said secondthyristor and primary winding. .Iaddend. .Iadd.
 10. An arc weldingcurrent supply unit as claimed in any of claims 2, 3 or 4 wherein saidswitching means comprises first and second thyristors and wherein saidat least one capacitor comprises first and second capacitors, said firstand second capacitors being alternately charged and discharged throughsaid primary winding by the alternate conduction of the second and firstof said two thyristors, wherein said first and second thyristors are inseries with one another and connected across said d.c. voltageintermediate source, said one and second capacitors are in series withone another and connected across said d.c. voltage intermediate source,and said primary winding is connected between the junction of said firstand second thyristors and the junction of said one and secondcapacitors. .Iaddend.